Monoclonal antibodies (MoAbs) have been generated or modified to study the origin, differentiation, and potential function of naturally-occurring murine cytotoxic leukocytes. A rat MoAb (4D11)k has been generated against a novel, 87 kDa, cell surface antigen on mouse large granular lymphocytes (LGL) with natural killer (NK) activity. This anitgen, LGL-1, can be detected on splenic or liverderived LGL from most st other types of lymphocytes or hematopoietic populations, and not LGL from other species. Cells electronically sorted for LgL-1 expression are highly enriched for LGL and for NK activity, whereas negatively sorted or MoAb + complement-depleted LGL-1- spleen cells contained little or no NK activity. Dual-color immunofluorescence revealed subsets of LGL-1+ cells which co-expressed Ly-1, whereas Cd8+ cells did not co-express LGL-1 can be detected on populations with lymphokine- activated killer (LAK) activity, but it appears that LGL-1 is not expressed on all precursors or effectors of LAk activity. In addition to the 4D11 MoAb, IgG variants of the anti-Qa-5 murine IgM MoAb, recognizing NK cells in some mice, were selected to determine whether IgG MoAbs might be more useful in vivo than the parental IgM. However, the IgG variant moAbs failed to bind Qa-5 antigen with sufficient affinity to pursue studies of cellular differentiation and function. A clonal variant producing both IgM and IgG1 was also isolated, which suggested that the sequence for isotype switching may not be parallel to the genomic configuration for the IgH genes. Finally, a MoAb to raf oncogene proteins has been generated and characterized. The antibody, STEGI-1, was generated against purified 30 kDa raf protein and recognizes the immunizing protein and 44-48 kDa protein in a v-raf/v-myc infected murine macrophage line and c-raf or a-raf transfected fibroblasts. Studies on the interaction of oncogene products in normal activation and differentiation of leukocyte subsets and the role of these oncogene products in malignant transformation are now being continued.